From German Low Germanschwul ‎(“sultry, hot and humid”) (17th century), adopted into Standard German as schwül in the 18th century. The older form without umlaut appears in Berlin dialect in the 19th century in the current sense, building on the slang term warm (as in warmer Bruder) for ‘homosexual’. The earliest attestation of this sense dates to 1847 (Paul Derks).